In this penultimate Bat-adventure, Dr. Cassandra releases six Arch-Criminals from Gotham State prison: The Riddler, The Penguin, The Joker, Catwoman, King Tut and Egghead. However none of them are played by the usual guest actors. For one thing it would be too expensive to feature them all at the same time. Note that their faces are never clearly seen, they have no lines (other than the Riddler's giggle and Penguin's squawk reused from earlier appearances) and Catwoman resembles the Julie Newmar version instead of season three's Eartha Kitt.
Howard Duff and Ida Lupino, who played husband and wife on this episode, were married off-screen, at the time, as well. The, by most accounts, loving but tempestuous marriage ended in 1984 after 33 years.
Writer Stanley Ralph Ross took the name Cassandra from a little girl by that name. She and her family lived next door to Ross, and she always wanted to be in a "Batman" episode, so he made her given name that of the character, Dr. Cassandra.
Originally the Alvino Ray Gun was to be named the Ronald Ray Gun, spoofing the name Ronald Reagan, at the time the Govornor of California. However, network censors objected to the naming, which was subsequently changed.